Abstract

Systemin, an octadeca-peptide isolated from tomato, is a signalling molecule involved in local and systemic wound response. It regulates the activation of more than twenty defensive genes in tomato plants in response to herbivore attacks. Systemin derives from the C-terminal region of a precursor of 200 amino acids, known as prosystemin. Prosystemin homologues have been found in other Solanaceae species such as potato (Solanum tuberosum), black nightshade (Solanum nigrum) and bell pepper (Capsicum annum), all members of the Solaneae subtribe, whereas they were not identified in tobacco, which belongs to the Nicotianae subtribe. Tobacco possesses two hydroxyproline-rich peptides released from the same precursor (preproTobHypSys) that show systemin-like activity. However, those peptides and their precursors are not similar in sequence to the tomato prosystemin. Even if it was previously showed that tobacco does not respond to tomato systemin when externally applied, it has been recently found that wild type tobacco cells are sensitive to systemin. Furthermore, a recent finding indicate that the constitutive expression of the tomato prosystemin in tobacco plants increases the expression of a number of proteins involved in plant defence against pathogens and oxidative stress. The aim of this project was to increase the understanding of the possible biotechnological role of the tomato prosystemin, with a particular emphasis to the exploitation of this precursor to increase the endogenous resistance against biotic stress. To this goal, the tomato prosystemin cDNA was expressed in tobacco and potato. Moreover, a mutated prosystemin cDNA lacking the 3’ terminal systemin encoding exon was also expressed to understand the possible function of the N-terminal region prosystemin precursor in the activation of the defence response. Transgenic tobacco plants expressing either of the prosystemin genes were already available. Transgenic potato plants were obtained after an Agrobacterium-mediated plant transformation. Tobacco and potato transformants were characterized and a group of genes involved in plant response against biotic stress was analysed by Real Time PCR. The modification in gene expression registered in MZ transgenic plants (expressing the tomato prosystemin cDNA) and PRO8 plants (expressing the deleted prosystemin) showed that HSP, GST, Pin II and TobHypSys, all related to plant response to stress, are over-expressed in tobacco transformants. Among the gene tested, GST gene was over-expressed only in MZ transgenic plants. Furthermore, in tobacco, the over-expression level induced by prosystemin of HSP and Pin II is similar to the effect of wounding. A bioassay with the pathogenic fungus Botrytis cinerea showed a moderate increase in resistance in the PRO8 tobacco plants. The expression of prosystemin gene in potato does not affect pathogen-related genes as GluB2 and PR1b nor the defence-related potato endogenous systemins, PotProsys and prePotHypSys genes. A effect was observed for Lox3 that was over-expressed in potato in both MZ and PRO8 transgenic plants. These data imply that the modification in gene expression in tobacco and potato is not only due to the Sys sequence and that the N-terminus is also involved.